A lithium ion secondary battery has been widely used for mobile electronic equipment such as mobile phones and laptop personal computers since it is a battery capable of attaining higher voltage and higher energy density and enables the battery to become smaller in size and lighter in weight compared to the conventional nickel-cadmium battery and nickel metal hydride battery. The lithium ion secondary battery is thought to further increase, in the future, in the opportunity of being utilized for onboard use in which the battery is incorporated into electric vehicles, hybrid electric vehicles and the like or industrial use such as electric power tools, and attaining further highly enhanced capacity and highly enhanced output has been eagerly desired.
The lithium ion secondary battery is composed of positive and negative electrodes having at least an active material capable of reversibly inserting/extracting lithium ions and a separator which is arranged in a container and separates the positive electrode from the negative electrode, the container being filled with a non-aqueous electrolytic solution.
The positive electrode is prepared by applying an electrode agent containing a positive electrode active material for a lithium battery, a conductive additive and a binding agent onto a metal foil current collector made of aluminum and the like and subjecting it to pressure forming. As the current positive electrode active material, a powder of composite oxides of lithium and a transition metal such as lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), or a ternary system material in which a portion of cobalt is substituted with nickel and manganese (LiMnxNiyCo1-x-yO2), and spinel type lithium manganate (LiMn2O4) has been used relatively frequently. In addition to these, metal oxides such as V2O5, metallic compounds such as TiS2, MoS2 and NbSe2, and the like have also been utilized.
In recent years, polyanionic active materials having high capacity have been attracting attention. The most advanced material in terms of development among the polyanionic active materials is olivine-based (phosphate-based) active materials with a high level of safety. Among the olivine-based active materials, lithium iron phosphate (LiFePO4) containing iron which is one of the abundant resources and is an inexpensive material has begun to be put into practical use. Moreover, lithium manganese phosphate (LiMnPO4) with a high level of output energy has also been attracting attention as a next-generation active material. As other polyanionic active material, silicate type active materials, and fluorinated olivine-based active materials among the olivine-based active materials have been attracting attention. The silicate type active material is characterized in that its discharge capacity per weight is higher than that of the olivine-based active material. The fluorinated olivine-based active material is characterized in that its voltage is higher than that of the olivine-based active material. These active materials are expected as a next-generation active material.
The negative electrode is prepared, as with the positive electrode, by applying an electrode agent containing an active material, a conductive additive and a binder agent onto a metal foil current collector made of copper or the like and subjecting it to pressure forming, and in general, as the active material for the negative electrode, lithium metal, lithium alloys such as a Li—Al alloy and Li—Sn, silicon compounds in which SiO, SiC, SiOC and the like are the basic constituent elements, conductive polymers prepared by doping lithium into polyacetylene, polypyrrole and the like, intercalation compounds prepared by allowing lithium ions to be incorporated into crystals, carbon materials such as natural graphite, artificial graphite and hard carbon, and the like have been used.
In the active materials currently put into practical use, a theoretical capacity of the positive electrode is far lower than that of the negative electrode, and hence it is indispensable to improve a capacity density of the positive electrode for increasing a capacity of the lithium ion battery. Thus, the practical realization of the polyanionic active material being a next-generation active material having a high capacity is desired. However, the polyanionic positive electrode active material is very difficult to be put into practical use since it is very low in electron conductivity. Thus, a technology of imparting electrical conductivity to the polyanionic positive electrode active material is desired.
In order to improve the electron conductivity in the positive electrode, a technique of adding a conductive additive to the electrode agent is employed. Examples of materials heretofore used as the conductive additive include graphite, acetylene black, Ketjen Black and the like. However, particularly, in the positive electrode active material having low electrical conductivity, it is insufficient only to add the conductive additive, and it requires a technique of directly forming a composite of an active material/conductive carbon material serving as a conductive additive.
Patent Document 1 discloses a technique in which a raw material solution of an olivine-based positive electrode active material and a polymer serving as a carbon source are mixed and the resulting mixture is subjected to spray drying and baking to prepare a composite. NON-PATENT DOCUMENT 1 discloses a technique in which lithium manganese phosphate is mixed in an aqueous solution of graphite oxide and the resulting mixture is heated to dry.
Patent Document 2 and NON-PATENT DOCUMENT 2 disclose a technique of heating/drying an aqueous solution in which a raw material of an olivine-based positive electrode active material and graphite oxide are dissolved.
Patent Document 3 discloses a technique in which a raw material of an active material is heated and baked, and mixed with graphite oxide, and the resulting mixture is reduced.
Non-Patent Document 3 discloses a technique in which a positive electrode active material is synthesized in the presence of a graphite oxide, and then reduced.